1. Field of the Invention
The present invention relates to a liquid crystal display device in which semiconductor devices using a thin-film semiconductor having crystallinity are arranged. In particular, the invention relates to an active matrix liquid crystal display device.
2. Description of the Related Art
In recent years, technologies for forming thin-film transistors (TFTs) on an inexpensive glass substrate have developed at high speed. This is due to increase in demand for the active matrix liquid crystal display device in which integrated circuits are formed on the same substrate.
In the active matrix liquid crystal display device, hundreds of thousands or more of pixels arranged in matrix form are respectively provided with thin-film transistors and charge entering/exiting from each pixel electrode is controlled by the switching function of the thin-film transistor.
The active matrix liquid crystal display device has a feature that integrated circuits are formed in which the thin-film transistors arranged in matrix form in the pixel area (i.e., pixel TFTs) are driven by driver circuit thin-film transistors (i.e., driver TFTs) that are provided around the pixel area.
However, to allow the active matrix liquid crystal display device to function properly, it is necessary that all the thin-film transistors in the pixel area operate normally. This is a factor of greatly reducing the yield of a manufacturing process of the active matrix liquid crystal display device.
Among causes of display defects in the pixel area are an operation failure of a pixel TFT and short-circuiting between wiring lines. In particular, the problem of short-circuiting between wiring lines should be solved thoroughly because it may cause a point defect or a line defect.
Among various types of electrodes and wiring lines provided in the active matrix liquid crystal display device are a gate line, a data line, a capacitor line, a black matrix, and a pixel electrode. These electrodes and wiring lines are made of conductive materials and insulated from each other by an interlayer insulating film.
However, for example, in a step portion where the coverage of an interlayer insulating film is poor, there may occur a case that sufficient insulation performance is not obtained because, for instance, a current path is formed between wiring lines due to insufficient thickness of the interlayer insulating film, a crack in the interlayer insulating film, or some other reason.
Process Leading to the Invention
FIGS. 1A and 1B show a general structure of an intersecting portion of a gate line and a data line and its vicinity in an active matrix liquid crystal display device in which a black matrix is formed on the same substrate as the pixel area. FIG. 1A is a sectional view taken along line A-A' in FIG. 1B.
Reference numerals 101-103 denote a glass substrate, an undercoat film, and an insulating film extending from a gate insulating film, respectively. A gate line 104 is formed on the insulating film 103, and a first interlayer insulating film 105 is so formed as to cover the gate line 104. A data line 106 is formed on the first interlayer insulating film 105.
A second interlayer insulating film 107 is so formed as to cover the data line 106, and a black matrix 108 is formed on the second interlayer insulating film 107. The black matrix 108 is covered with a third interlayer insulating film 109, and a pixel electrode 110 is formed thereon. A thin film 111 as an alignment film is formed on the pixel electrode 110.
FIG. 1B is a top view of an intersecting portion 100 of the gate line 104 and the data line 106. In FIG. 1B, the gate line 104 and the data line 106 are drawn by broken lines because they exist below the black matrix 108.
As mentioned above, the pixel electrode 110 is formed above the black matrix 108 with the third interlayer insulating film 109 interposed in between. In this structure, a capacitor is formed by the black matrix 108 and the pixel electrode 110. The present inventors utilize this capacitor as an auxiliary capacitor.
Therefore, to increase the capacitance of the auxiliary capacitor, it is desirable that the overlapping portion of the black matrix 108 and the pixel electrode 110 be as wide as possible. However, if the pixel electrode 110 is so formed as to overlap with the intersecting portion 100, the following problems will occur.
In FIG. 1A, reference numerals 112 and 113 denote spacers to determine the cell gap of the liquid crystal display device. The spacers 112 and 113 are interposed between and pressed by the active matrix substrate (which means the glass substrate 101 and the device structure formed thereon collectively) and an opposed substrate 114.
The spacers 112 and 113 are arranged at a certain density on the surface of the active matrix substrate (or the opposed substrate). For example, in the case of a small liquid crystal display device having a diagonal size of about 3 inches, they may be distributed at a density of several tens of pieces per square centimeters. However, as the size of the liquid crystal display device increases, the density needs to be increased in a range of several tens to several hundreds of pieces per square centimeters.
Since the spacers exist at a given density in the pixel area of the active matrix display device, a spacer may be located at the intersecting portion 100 of the gate line 104 and the data line 106 at a certain probability.
In this case, since the intersecting portion 100 of the gate line 104 and the data line 106 considerably protrudes from the flat portion, the spacer 112 placed thereon receives much stronger pressure than the spacer 113 which is placed on the flat portion.
The strong pressure exerted on the spacer 112 in turn imposes a heavy load on the underlying laminate structure, to possibly cause break-off portions 115 such as a crack in the third interlayer insulating film 109. As a result, a current path is formed between the black matrix 108 and the pixel electrode 110, so that they no longer has the function of forming an auxiliary capacitor. This may causes an image display defect.
It is easily understood that the above defect is caused by the structure in which the pixel electrode 110 is laid on the highest portion of the step as shown in FIG. 1A, and hence the problem can be solved by avoiding such a structure.
On the other hand, to increase the capacitance of the auxiliary capacitor, it is necessary to make the overlapping portion of the black matrix 108 and the pixel electrode 110 as wide as possible. In the above circumstances, the inventors thought that it would be proper to provide a structure shown in FIG. 2A.
FIG. 2A is a top view corresponding to FIG. 1B. FIG. 2A therefore uses the same reference numerals as FIG. 1B.
In this structure, the pixel electrode 110 does not overlap with the gate line 104 nor the data line 106. That is, the pixel electrode 110 is not laid on the highest portion of the intersecting portion 100, so that there should not be formed any current path between the black matrix 108 and the pixel electrode 110 due to the above-mentioned pressure exerted from a spacer.
Actually, however, there may occur a case that the pixel electrode 110 is laid on the highest portion of the intersecting portion 100 due to a patterning error, i.e., insufficient patterning accuracy, as shown in FIG. 2B.
In this state, defective portions 115 may occur in the third interlayer insulating film 109 due to the above-mentioned pressure from a spacer, disabling the black matrix 108 and the pixel electrode 110 to form an auxiliary capacitor.
Although the safest measure is to form the pixel electrode 110 with a large margin in light of the patterning accuracy, the increase in margin is not desirable in view of a future situation where it will become difficult to secure necessary capacitance of the auxiliary capacitor as the miniaturization of wiring lines proceeds.